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Protein synthesis in eukaryotes is carried out by ribosomes, large RNA-protein complexes consisting of a small and a large subunit. In this work we present a mathematical model for cellular growth comprising both protein production and ribosome synthesis, properly accounting for both small and large subunits dynamics. The qualitative analysis of the model is carried out according to a simplifying assumption on the proportion of the two ribosomal subunits in stationary growth conditions; such hypothesis is based on a reasonable biological ground. Conditions are given on the model parameters in order to ensure exponential growth and the corresponding growth rate is straightforwardly computed from the model parameters. These results are validated by numerical simulations carried out according to a set of biologically meaningful model parameters. The modified model is better suited to host molecular blow-up of ribosomal synthesis and cell growth within a modular whole-cell model able to act as a scaffold connecting metabolism, growth and cycle.

Rhodococcus members excrete secondary metabolites, especially compounds which act as biosurfactants. In this work, we demonstrated the ability of Rhodococcus opacus R7 to produce a novel bioactive compound belonging to the class of biosurfactants with antimicrobial properties during the growth on naphthalene. Chemical and biochemical analyses of the isolated compound demonstrated that the biosurfactant could be classified as a hydrophobic peptide. The ESI-full mass spectrometry revealed that the isolated biosurfactant showed a molecular weight of 1292 Da and NMR spectra evidenced the composition of the following amino acid residues: Ala, Thr, Asp, Gly, Ser. Surfactant activity of the R. opacus R7 compound was quantified by the critical micelle dilution (CMD) method and the critical micelle concentration (CMC) was estimated around 20 mg L−1 with a corresponding surface tension of 48 mN m−1 . Moreover, biological assays demonstrated that R. opacus R7 biosurfactant peptide exhibited antimicrobial activity against Escherichia coli ATCC 29522 and Staphylococcus aureus ATCC 6538 with the minimum inhibition growth concentration (MIC) values of 2.6 mg mL−1 and 1.7 mg mL−1, respectively. In this study for the first time, a hydrophobic peptide with both biosurfactant and antimicrobial activity was isolated from a bacterium belonging to Rhodococcus genus.

We investigated the effect of beta-amyloid (A beta) (25-35), a cytotoxic fragment of A beta pepticle, on lipid metabolism and protein ubiquitination in cultured rat hippocampal neurons. After treatment with A beta under conditions leading to apoptotis, as assessed by caspase activity assay, the total cell mass of lipids changed following a biphasic behavior, with an increase that reached a maximum after 16 hr of treatment, followed by a decrease. The increase at 16 hr was 15.3% in the case of phospholipids and 103.0% in the case of gangliosides and was due to enhanced biosynthesis as confirmed by increase of radioactivity incorporation (phospholipids +52.0%, gangliosides +193.1%) in cells fed with tritiated palmitic acid. No change with respect to cholesterol was observed. Strikingly, under these conditions, the ubiquitination state of cell proteins strongly increased. These effects were not observed with the (35-25) reverse sequence pepticle. Similarly to A beta, lactacystin treatment increased lipid synthesis and protein ubiquitination; only lactacystin, and not A beta, induced a strong decrease of proteasome chimotrypsin activity. These results suggest that A beta enhances protein ubiquitination, without inhibiting proteasomal activity, and lipid synthesis. These results may shed new light on the mechanisms of A beta toxicity. (c) 2007 Wiley-Liss, Inc.